Put your mouse over any English word. Less than a second, KitaDic will fetch The Meaning of the word under your mouse from Wiktionary.org , and display it in Post-it-like window. Further, it provides links to the entry in en.Wikipedia.org, Answers.com.

It provides ultra-easy on/off switch on the bottom-right corner of the FF window, for in case that you feel the dictionary is cumbersome.